Hitching a Ride with a Carnivore

Animals can help disperse seeds, but did you know that sometimes a seed-eater gets eaten by the predator, and that’s actually a good thing for the seeds in the gut of the seed-eater? Read more below:

Plants can’t move. That may seem like an obvious statement, but it has a lot of consequences for how plants live their lives and the kinds of adaptations that they have. Not being able to move is particularly problematic when they have offspring. For example, if a plant’s offspring grew next to the parent plant, the parent and offspring would probably end up competing with each other for space, water, and nutrients for the rest of their lives. So parent plants have to get creative in transporting their offspring elsewhere, even though the parents themselves cannot move. This movement away from the parent plant is called dispersal. Some plants have solved this problem by manipulating animals to transport their seeds. For example, their seeds could be contained within a fleshy, edible fruit that would then be eaten by a foraging animal. After the edible fruit was digested, the seeds could be deposited some distance away, thus solving the parent plant’s problem. Having seeds transported in this way comes with the added benefit that animal poop can be highly nutritious, which is great for a seedling just starting to grow. However, it’s pretty common that seeds don’t just germinate wherever they’re deposited: seeds are often transported by multiple animals or other means such as wind or water. For example, a cherry eaten by a bird could have its seed first deposited by the bird and then transported by ants where it then grows into a cherry tree.

We were interested in seed dispersal because this process can get quite bizarre. Many animals that eat seeds or fruit fall prey to predators. If the prey had recently eaten, they could still have seeds or fruit in their gut when they were killed by the predator. This means that the seeds that started out being eaten and then dispersed by one animal, ended up in the gut of a predator instead. In our cherry tree scenario, this could happen if the bird ate the cherry only to be consumed by a fox afterward. The cherry seed could then hitch a ride with the fox instead of the bird. The process of a seed being transported in the gut of multiple animals, such as first by a prey animal that was then eaten by a predator, is called diploendozoochory.

We wanted to know how widespread this phenomenon was and how important it was for plant populations. After reviewing scientific literature, we found that this kind of predator-assisted seed dispersal was first described by Charles Darwin in 1859. Since then, there have been other sporadic observations and we found that there is potential for this phenomenon to occur in many habitats and species. These studies showed that seeds consumed by prey that were eaten by predators may be moved greater distances than seeds deposited by the prey alone. Predators and prey may travel through different kinds of habitats, which means that seeds can end up in different places depending on who deposits them. Some seeds have particularly thick shells, which must be cracked open for the seedlings to grow. These plants can benefit from the wear and tear of passing through the guts of two animals, making them better able to germinate than if they had passed through the gut of the prey alone. It’s even possible that some plants have evolved specifically to take advantage of these predator-specific behaviours, in other words their seeds have evolved counting on the prey being eaten by a predator. However, these different factors are like pieces in a puzzle: to fully understand the big picture of how they affect plant populations, we need to know how all of these pieces fit together. So far, studies have only looked at small parts of the puzzle, and no study has put all of the pieces together to see the overall importance of this phenomenon for plant populations or its role in seed evolution.